Static and dynamic responses of underwater structures which might be used as LNG storages or underwater stations have been studied by the authors recently for one of the structural feasibility studies. Furthermore the authors have been studying dynamic characteristics of axisymmetric underwater shells with the effect of added mass of water experimentally and numerically in order to study the dynamic problems such as seismic response. In the present report, firstly we have developed a numerical code, that is, the sea are used for analyzing diffraction problem of underwater axisymmetirc shell in the sea. The fluid velocity potential distribution of shell surface is obtained by our code, and dynamic characteristics of axisymmetric underwater shell in waves have been examined and discussed.